http://www.authorstream.com/ -- a wide collection of PowerPoint presentations. Lots of student work, but some other collections, too. Good search engine.
http://www.slideshare.net/ -- the most widely-used PPT sharing site. Lots of presentations from businesses; some student work, too.
http://www.slideshare.net/ -- the most widely-used PPT sharing site. Lots of presentations from businesses; some student work, too.
Some others (thanks to Larry Ferlazzo)
Authorstream
Slideboom
280 Slides
My Brainshark
PowerShow
Slideserve
MyPlick
SlideSix